82 
PHARMACEUTICAL BOTANY 
facing toward the exterior. Stems of most Monocotyledons and 
Horsetails. 
II. Open collateral, consisting of a mass of xylem facing toward 
the pith and a mass of phloem facing toward the exterior and sepa- 
rated from each other by a cambium. Stems and leaves of Dicoty- 
ledons and roots of Dicotyls and Gymnosperms of secondary growth. 
Fig. 32.- — Cross-section through a portion of a root of Acorus calamus. A. 
Cortical parenchyma; B, endodermis; C, pericycle; E, phloem; F, xylem. At 
F, Y, are large tracheal tubes, which were formed last, the narrow tubes near 
the periphery of the xylem being formed first. At the center of the root, within 
the circle of the vascular bundle, occur thin-walled parenchymatous pith cells. 
(From Sayre after Frank.) 
III. Bicollateral, characterized by a xylem mass being between 
an inner and an outer phloem mass. There are two layers of cam- 
bium cells, one between the xylem and inner phloem mass, the other 
between the xylem and outer phloem mass. Seen chiefly in stems 
and leaves of the Cucurbitacece and Solanacece. ' 
IV. Concentric, characterized by a central xylem mass surrounded 
by a phloem mass or vice versa. No cambium present. 
